This invention relates to an anti-lock control system for vehicle wheel brakes in which the actuator for establishing the braking pressure is provided by the operation of a DC torque motor and, particularly, such a system wherein the torque motor current is utilized for the measurement and control of the braking pressure.
When the brakes of a vehicle wheel are applied, a braking force between the wheel and the road surface is generated that is dependent upon various parameters including the road surface condition and the amount of slip between the wheel and the road surface. For a given road surface, the force between the wheel and the road surface increases with increasing slip values to a peak force occurring at a critical wheel slip value. As the value of wheel slip increases beyond the critical slip value, the force between the wheel and the road surface decreases. Stable braking results when the slip value is equal to or less than the critical slip value. However, when the slip value becomes greater than the critical slip value, braking becomes unstable resulting in sudden wheel lockup, reducing vehicle stopping distance and deterioration in the lateral stability of the vehicle.
Numerous wheel lock control systems have been proposed to prevent the wheels from locking while being braked. Typically, these systems prevent the wheels from locking by releasing the applied brake pressure when an incipient wheel lockup condition is sensed. After release of the brake pressure, the wheel recovers from the incipient wheel lockup condition after which brake pressure is re-applied. Various criteria have been used in order to determine when the brake pressure should be released and thereafter re-applied in order to maintain stable braking. For example, the values of wheel deceleration, acceleration and slip have been used in various systems to establish the times at which the brake pressure should be released and re-applied. Other systems have been proposed that also include measurements of the brake pressure actuator position and/or the brake pressure. Each of these systems require transducers for measuring the brake pressure and/or the position of the brake pressure actuator.